Scientists have genetically modified a human embryo for the first time. Junjiu Huang, a gene-function researcher at Sun Yat-sen University in Guangzhou, China, confirmed that his team has modified the gene responsible for a potentially fatal blood disorder in non-viable embryos - ones that cannot result in a live birth.

They reported their work, which up until now was merely a rumor, in this week's online journal Protein & Cell.

The results will no doubt reignite an already highly contentious debate between those who think editing the genome of a human before it's born could prevent genetic disease and those who believe the unpredictable effects could be devastating to humankind.

Huang's team used a technique well known in genetic science called CRISPR/Cas9. Essentially, scientists inject an embryo with an enzyme of the same name that can be programmed to bind and splice DNA at a specific gene location. During the process, another molecule is introduced to that location to repair genetic damage.

Until now, no one has reported trying this technique on a human embryo.

The Chinese team tried it on not just one human embryo, but 86. These were obtained from fertility clinics and were unviable because they all contained an extra set of chromosomes after being fertilized by two sperm.

After injecting the enzyme into the embryos, the team waited 48 hours, enough time for the enzyme to splice the gene, for the molecule to replace the missing DNA and for the single-celled embryos to grow to about eight cells each.

Seventy-one embryos survived, and of those, 54 were tested and of those, only 28 showed a successful splice. Even more disconcerting, only a fraction of the 28 embryos contained the replaced molecule.

Huang admitted that the experiment was not a success - closer to a 100 percent success rate would be needed for such a procedure - and as a result, the team stopped, calling the technique immature.

Even with a 100 percent success rate, however, plenty of scientists warn that this kind of technique would be unethical. Genetic changes to an embryo are passed along to offspring and the effects on future generations are unpredictable.

But Huang is not shelving the experiments. According to Nature, he will consider alternative strategies for tweaking and/or administering the enzymes.

And apparently he isn't alone. David Cyranoski and Sara Reardon reports in Nature News that at least four groups in China are pursuing gene editing in human embryos.

In Greek mythology, chimeras were vicious monsters feared by many. This fire-breathing animal had the head and body of a lioness, with a goat head protruding from her back and the tail of a snake. Today, “chimera” refers to an animal that has two or more different sets of genetically distinct cells working together. Remember the mouse with the ear on its back?
The movie "Splice" showcases a chimera experiment gone horribly wrong: scientists create a human-animal hybrid that becomes evil and goes completely out of control.
While the movie is obviously science fiction, chimera experiments with human cells are not, and real life scientists have been conducting them for decades. We take a look at a few that have been successful in the past and how they’re advancing medicine.

The first successful human-animal chimeras were reported in 2003.
Chinese researchers at the Shanghai Second Medical University successfully fused human cells with rabbit eggs. They were allowed to develop the eggs for several days in a petri dish before the embryos were harvested for their stem cells.
Their hope was that this process could one day be used to grow cells or tissues for transplantation.

A year after the successful Chinese chimera experiment, researchers at the Mayo Clinic in Minnesota announced they had created pigs with human blood pumping through their veins.
What was startling about the animal is not only did the pig blood cells flow with human cells, but some of the cells merged together, creating pig-human cell hybrids. Scientists said this experiment can give them a better understanding of how viral infections can pass from animals to humans such as HIV and various others.

One of the efforts behind creating chimeras is to generate animal specimens that could grow human organs to be farmed for transplantation.
In 2007, scientists at the University of Nevada-Reno announced they could grow livers made up of 20 percent human cells in sheep. Dr. Esmail Zanjani injected either human adult stem cells derived from bone marrow, or human embryonic stem cells, into growing sheep fetuses. Zanjani said he uses sheep because the circulation systems of sheep and humans are similar.

How do you develop treatments for liver infections and diseases only humans can get? Salk Institute researchers came up with one solution in February 2010.
Using a mouse that was having liver problems of its own, the researchers replaced its liver with one that was made up of 95 percent human cells to study treatments for Hepatitis. Shown here is a cluster of mouse liver cells that have been replaced with human cells (shown in green).
Typically, small animals can't be infected with Hepatitis, only humans and chimps can, but this "humanized" mouse not only became infected, it successfully responded to drug treatments. Scientists believe this experiment could open doors to finding cures for other human liver infections such as malaria.

British researchers were given approval to conduct human-animal hybrid research in 2008, a decision that researchers touted would give them the ability to possibly find a cure for Parkinson’s disease.
Before, only human cells were allowed to be injected into human eggs, but the researchers argued that animal eggs are much more available. After given permission, researchers went to work using cow eggs.
The nucleus of the cow egg -- the source of most DNA and shown here in blue -- was removed, and replaced with the nucleus of a human cell such as a skin cell. Once the egg was allowed to develop and multiply it would become a early-stage cloned embryo called a blastocyst. Scientists could then extract the stem cells from this blastocyst to use in disease treatments.

Allergic to cats? Then you’ll appreciate this experiment.
The feline Fel d 1 protein found in cat saliva contains an allergen that affects humans. When cats lick themselves, the saliva on their fur dries and turns into dust.
In April 2005, scientists at the University of California created a human-cat hybrid when they fused the Fel d 1 protein with a human protein known to suppress allergic reactions. The feline protein would bind to immune cells that would cause the reaction and the human protein would tell the immune cells to calm down.
When tested in mice, the chimeric protein stifled the allergy, and researchers hope they can be used in the future to treat allergy sufferers.

Irving Weissman, Stanford University professor and cofounder of the biotech company StemCells Inc., was granted permission by Stanford to create a mouse-human hybrid in 2005.
Weissman and his team transplanted human-brain stem cells into the brains of mice with the intention to study neurodegenerative diseases such as Parkinson's and Alzheimer's. In his initial experiment, the human cells only made up less than 1 percent of the mouse brain. Shown here is an isolated mouse brain cell.
In 2010, Stanford researchers announced they transformed mouse skin cells into fully functional neurons in a laboratory dish for the first time. They also announced in May that they successfully used mouse stem cells to develop sensory hair cells, which could combat human hearing loss.

We share over 98 percent of our DNA with chimpanzees, so would it be possible to create a human-chimp hybrid: a "humanzee," also called a "chuman" or "chumanzee"?
In the 1920s, a Soviet biologist Ilia Ivanov artificially inseminated female chimps with human sperm, but the pregnancies didn't take. A chimp named Oliver became famous in the 1970s after it was thought he could be a human-chimp hybrid, because he walked upright. However, genetic testing in the 90s proved he was a chimp.
Several researchers and citizens see such experiments has highly immoral and there is no known evidence of a human-chimp hybrid.